Measurement of Direct Emission in the KL->pi+pi-gamma Decay Mode

TL;DR

This study measures the contributions of different photon emission mechanisms in KL->pi+pi-gamma decays, providing precise values for the M1 amplitude and setting limits on E1 emission, enhancing understanding of CP violation and decay dynamics.

Contribution

The paper presents the first detailed measurement of the M1 direct photon emission amplitude and form factor, along with an upper limit on the E1 amplitude, in KL->pi+pi-gamma decays.

Findings

Measured |g_M1| = 1.198 +/- 0.035(stat) +/- 0.086(syst)

Determined a_1/a_2 = -0.738 +/- 0.007(stat) +/- 0.018(syst) GeV^2/c^2

Set an upper limit |g_E1| < 0.21 (90% CL)

Abstract

In this paper the KTeV collaboration reports the analysis of 112.1*10^3 candidate KL->pi+pi-gamma decays including a background of 671+/-41 events with the objective of determining the photon production mechanisms intrinsic to the decay process. These decays have been analyzed to extract the relative contributions of the CP violating bremsstrahlung process and the CP conserving M1 and CP violating E1 direct photon emission processes. The M1 direct photon emission amplitude and its associated vector form factor parameterized as |g_M1|(1+ (a_1/a_2)/(M(rho)^2-M(K)^2+2M(K)*E(gamma)) have been measured to be |g_M1|=1.198 +/- 0.035(stat) +/- 0.086(syst) and a_1/a_2 = -0.738 +/- 0.007(stat) +/- 0.018 (syst) GeV^2/c^2 respectively. An upper limit for the CP violating E1 direct emission amplitude |g_E1| < 0.21 (90% CL) has been found. The overall ratio of direct photon emission (DE) to total photon emission including the bremsstrahlung process (IB) has been determined to be DE/(DE +IB) = 0.689 +/- 0.021 for E(gamma) > 20 MeV.